1. Field of the Invention
The present invention relates to a novel acid generator which is suitably used, especially as an acid generator for a resist composition, a chemically amplified resist composition using the same, and a patterning process.
2. Description of the Related Art
In recent years, as LSI progresses toward a high integration and a further acceleration in speed, a miniaturization of a pattern rule is required. Deep-ultraviolet lithography and vacuum ultraviolet lithography are thought to hold particular promise as the next generation microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is the technique essential to ultra-fine microfabrication capable of achieving a size of 0.13 μm or less.
ArF lithography has started being partially used from production of a 130 nm node device, and used as a main lithography technique from production of a 90 nm node device. At first, a 157 nm lithography with a F2 laser has held promise for the next technique of 45 nm node lithography. However, a delayed development caused by various problems has been pointed. Thus, ArF immersion lithography has been proposed suddenly. In the ArF immersion lithography, a liquid having a higher refractive index than air, such as water, ethylene glycol, or glycerol, is placed between a projection lens and a wafer to set the numerical aperture (NA) of the projection lens to 1.0 or more and achieve a high degree of resolution (see, Non-Patent Document 1).
To prevent a precise and expensive optical material from deteriorating, the ArF lithography requires a resist composition having a high sensitivity capable of getting sufficient resolution even by a small exposure dose. As a strategy for the resist composition, it is most common that a component having a high transparent at a wavelength of 193 nm is selected. For example, a polyacrylic acid and derivatives thereof, a norbornene-maleic anhydride alternating polymer, polynorbornene, a ring-opening metathesis polymer, and a hydrogenated ring-opening metathesis polymer are proposed as a base resin. This achieves some results in terms of increasing transparency of a resin itself.
Various photoacid generators have been investigated. When a photoacid generator generating an alkane or an arene sulfonic acid, which is used for a chemically amplified resist composition using the conventional KrF excimer laser as the light source, is used as a component of the ArF chemically amplified resist composition, the acid strength is not sufficient to cut the acid labile group of a resin, a resolution cannot be achieved, or a sensitivity is low. Thus, the resist composition is not suitable for production of a device.
For this reason, as a photoacid generator in the ArF chemically amplified resist composition, photoacid generators generating a perfluoroalkanesulfonic acid having a high acid strength are typically used. The photoacid generators generating a perfluoroalkanesulfonic acid have been developed for a KrF resist composition. For example, Patent Documents 1 and 2 describe photoacid generators generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid. Further, as a novel acid generator, acid generators generating a perfluoroalkyl ether sulfonic acid are suggested in Patent Documents 3 to 5.
On the other hand, perfluorooctanesulfonic acid or derivatives thereof are known as its acronym PFOS, and has problems of stability (non-degradability) derived from a C—F bond, bioconcentration due to hydrophobicity and lipophilicity, and accumulation. Environmental Protection Agency (EPA) has adopted Significant New Use Rule, listing 13 PFOS-related chemical substances and further 75 chemical substances although their use in the photoresist field is excluded. Further, it has been proposed to apply Significant New Use Rule to 183 chemical substances such as a perfluoroalkanesulfonic acid or a derivative thereof.
To cope with the problems of PFOS, various companies have developed a partially fluorine-substituted alkanesulfonic acid having a lower fluorine substitution ratio. For example, Patent Document 6 discloses development of an α,α-difluoroalkanesulfonic acid salt from an α,α-difluoroalkene and a sulfur compound, and a resist composition containing a photoacid generator generating such a sulfonic acid upon exposure, specifically di(4-tert-butylphenyl)iodonium=1,1-difluoro-2-(1-naphthyl)ethanesulfonate. Patent Document 7 discloses development of an α,α,β,β-tetrafluoroalkanesulfonic acid salt from an α,α,β,β-tetrafluoro-α-iodoalkane and a sulfur compound, a photoacid generator generating such a sulfonic acid, and a resist composition. Patent Document 3 discloses a photoacid generator having a difluorosulfoacetic acid alkyl ester (e.g., 1-(alkoxycarbony)-1,1-difluoromethane sulfonate), or a difluorosulfoacetic acid amide (e.g., 1-carbamoyl-1,1-difluoromethane sulfonate), but does not describe a synthesis method thereof. Patent Document 8 discloses a photosensitive composition containing a compound generating a partially fluorinated-alkanesulfonic acid having a sulfonylamide structure derived from perfluoroalkylene disulfonyl difluoride.
The substances described in Patent Documents have a lower fluorine substitution ratio. However, since the substances have a hydrocarbon skeleton of which the basic skeleton is hard to be degraded, and do not have a substituent which is easily degraded like an ester group, the substances have low degradability. Further, the molecular design in which the size of alkanesulfonic acid is changed is limited. Moreover, the substances have problems such as expensive fluorinated-starting materials.
In addition to degradability and acid strength of photoacid generators, there are many problems. For example, a pattern layout has become finer in recent years, and fluctuation of pattern line width (line width roughness (LWR)) becomes a problem. In processing a gate electrode portion during production of an LSI circuit, a low LWR causes a problem such as leakage current, and thus the electrical characteristics of a transistor deteriorates. The low LWR is thought to be caused by various factors. A main factor thereof is the low affinity of a base resin and a developer, that is, the low solubility of a base resin to a developer. Since a carboxylic acid protecting group conventionally used is a bulky tertiary alkyl group and has a high hydrophobicity, the solubility is low. When a high resolution is required as in formation of microscopic channels, a large LWR leads to an uneven size. A known approach for reducing LWR is to increase the amount of a photoacid generator to be added (Non-Patent Document 2). However, this approach does not achieve sufficient effect since characteristics such as exposure dose dependency, mask fidelity, and pattern rectangularity sometimes deteriorate extremely. Not only increasing the amount of a photoacid generator, but also uniformly dispersing the photoacid generator is required for improvement of LWR.
As the circuit line width is reduced, the influence of contrast degradation by acid diffusion becomes more serious for the resist composition. This is because the pattern size approaches the diffusion length of acid, and this causes a decrease of mask fidelity and a degradation of pattern rectangularity. Accordingly, to obtain more benefits from a reduction of the wavelength of light source and an increase of NA, the resist composition is required to increase dissolution contrast or suppress acid diffusion as compared with the conventional material.
Patent Document 9 discloses alkanecarbonyloxy-1,1,3,3,3-pentafluoropropane sulfonate having 1 to 20 carbon atoms or arenecarbonyloxy-1,1,3,3,3-pentafluoropropane sulfonate having 1 to 20 carbon atoms, such as triphenylsulfonium 2-(adamantane-1-carbonyloxy)-1,1,3,3,3-pentafluoropropane sulfonate, but is not sufficient to control diffusivity and achieve a low LWR.
Patent Documents 10 and 11 disclose photoacid generators generating a partially fluorinated-alkanesulfonic acid having a polycyclic hydrocarbon group, but cannot achieve sufficient resist performance. Since each photoacid generator is a labile ester of difluoroacetic acid, the storage stability of a resist composition containing the photoacid generator may be decreased.
The depth of focus tends to decrease due to a reduction of the wavelength of a light source for light lithography in a device process. Even when a light source with a short wavelength is used, a depth of focus capable of resolving a wide range is required. While a miniaturization of a pattern rule is required, excellent sensitivity, substrate adhesion, and etching resistance are required. In addition, an improvement of LER and an increase of depth of focus (DOF) are necessary without degradation of a resolution.
In immersion lithography, there are problems. Minute water droplets are left on a resist wafer after the immersion exposure, causing damages to the resist pattern profile, collapse or deform into a T-top profile to the developed resist pattern. Therefore, a patterning process which can form a good resist pattern after development in immersion lithography is required.